The present invention generally relates to integrated circuits, and more particularly relates to digitally spreading a clock spectrum in digital circuits.
The spreading of a clock spectrum generally includes spreading the power of clock pulses over a range of frequencies. FIG. 1 is a simplified schematic of a first clock train 100 and a spread-spectrum clock train 105. The pulses in spread-spectrum clock train 105 are spread-spectrum pulses that may be generated from the clock pulses in the first clock train 100. The clock edges of the spread-spectrum pulses move in and out as indicated by the arrows in FIG. 1. A spread-spectrum clock train is typically generated for applications in which jitter sensitivity is generally low and/or in which lowering peak power of clock pulses is desired.
Traditional circuits configured to spread the spectrum of a clock train often include both digital and analog circuits, which typically include a digital modulator and an analog phase interpolator. The digital modulator is configured to receive a clock train (e.g., the first clock train) from a clock generator. The clock train may be a digital clock train. The digital modulator may be configured to modulate the clock train using a dithering waveform to generate a dithered waveform. The dithered waveform may then be transferred to the analog-phase interpolator that is configured to phase interpolate the dithered waveform. Based on the phase interpolation of the dithered waveform, the analog-phase interpolator is configured to generate and output a spread-spectrum clock train.
These traditional circuits configured to spread the spectrum of a clock train have a number of inherent shortcomings especially for low power circuit applications. For example, typical digital modulators configured to modulate a clock train typically operate at the clock frequency of the clock train, which is typically the relatively high frequency of a free running clock (e.g., generated by a crystal oscillator). As traditional digital modulators are configured to operate at a free running clock frequency, these digital modulators tend to draw relatively high current.
Not only do the digital modulators included in these traditional circuits typically draw relatively high current, the analog-phase interpolators in these traditional circuits also typically draw relatively high current. For example, an analog-phase interpolator may draw as much as a digital modulator. Moreover, as these traditional circuits typically include analog devices (namely, an analog-phase interpolator), the shape of the spread-spectrum clock pulses and the amount of power reduction of these clock pulses is generally limited by the linearity of the analog-phase interpolator. Analog-phase interpolators having relatively high linearly are relatively costly to design and manufacture, and are relatively large. Also, as these traditional circuits often include both digital circuits and analog circuits, these traditional circuits tend to take up relatively large amounts of die space, which tends to make these circuits relatively costly to manufacture.
Therefore, new circuits are needed that are configured to spread the spectrum of a clock train, that draw relatively less current than traditional circuits that are configured to provide this function, and that take up relatively less die area than traditional circuits.